Motor Driver Circuit

Discussion in 'The Projects Forum' started by peter_morley, Nov 24, 2012.

  1. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    I am using 6 npn transistors and am having trouble getting enough voltage across the motor at start up. If I give the wheels a push and they start turning, the voltage increases from about 700 mV to 4-5 Volts. I assume this has to do with the increasing impedance of the motor. With this setup I can make the wheels move fine when they are not on the floor but when I put the wheels on the floor the wheels do not have enough torque to get going. I estimated that the start up load of the motor is on the order of 1-2 ohms when the voltage across it is 700mV. When the wheels start turning the voltage goes up and the current stays about the same so I assume that the resistance of the motor is variable. My 2 bit input comes from the GPIO pins of a gumstix overo board in which logic high is 1.8V and logic low is 0V.
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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    If the start up load on the motor is 1-2 ohms at 12VDC you are asking those poor little transistors to supply over 10 amps !
    Need much higher current transistors or Mosfets. N Mosfets require extra gate voltage on the high side to turn fully on.

    What is the motor voltage ?
     
    Last edited: Nov 24, 2012
  3. peter_morley

    Thread Starter Member

    Mar 12, 2011
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  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    The datasheet shows the motors draw 2.9 amps stall current.
    The running current is much lower, depending on weight of the machine.
    Mosfets might be a better choice. Mosfets or transistors should handle at least twice stall current, 6 amps or more.

    Couple more concerns:
    If both inputs are off, all transistors are on which also makes then heat up.
    It's better to have one in each 'stack' off when stopped.
    Diodes should be installed across the transistors for protection from voltage spikes.

    Google Motor H-bridge circuits - lots of examples out there.
     
  5. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    The issue seems to be that I can't apply a large enough voltage at start because the resistance of the motor is about 1-2 ohms. Lost!!!
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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  7. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    Do you have any suggestions for MOS transistors because I don't have much experience with high current and high voltage
    rated MOS. Thanks
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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  9. Audioguru

    New Member

    Dec 20, 2007
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    Since you used weak little transistors then I changed the motor to a tiny 100 ohms one.
    Then I fixed the circuit so the motor runs pretty well with 9V and the remaining 3V is wasted.
     
  10. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    I switched the 4 2n3904 bridge transistors with TIP29 power transistors and it worked great! I think the 2n3904 could not supply enough current or the other reason is that the base emitter voltage of the TIP29 might be lower to turn the transistor into forward bias on mode. I am excited to make a robot now!
     
  11. John P

    AAC Fanatic!

    Oct 14, 2008
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    Are you really using those 100 ohm resistors as shown? If Q2 switches its collector to Gnd, then R7 will be burning up almost 1.5 Watts. Even if it's a big enough resistor to do that (and is it?) it's a totally unnecessary waste of power. If you're running off a battery, I can't believe you'd ever want to do that.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    100 ohms is too high. The output transuistors must start the motor with 2.9A so the base current must be 290mA to 363mA.
     
  13. tubeguy

    Well-Known Member

    Nov 3, 2012
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    It's exciting to get a project working. You may know this, but you should put heatsinks
    on the TIP 29's and the tabs must be insulated from the heat sink and each other because the are connected to the transistor collector. Yes - two collectors do go to 12VDC - so technically they can be connected.
     
  14. John P

    AAC Fanatic!

    Oct 14, 2008
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    Let's go near the minimum there for simplicity, 300mA.

    Then 12V * .3A = 3.6W!

    It may turn the motor, but I think this isn't a good design. At least make the transistors Darlingtons, or better still, FETs.
     
  15. tubeguy

    Well-Known Member

    Nov 3, 2012
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    And, you are asking a 2n3904 to supply 300 ma. Per the datasheet the absolute max continuous current is 200ma. Just not good design practice.

    I noticed an error in the MOSFET driver circuit posted above,
    Corrected Version:
    Mosfet N-P Bridge 1.GIF
     
  16. Adesoji Aderemi

    New Member

    Oct 27, 2012
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    Sorry guys if I make you laugh. What do you still think about amplidynes old. They supply high current.
     
  17. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Everyone on the forum is here to learn and/or share info. Everyone was a noob - newbie - beginner at one time. :)
    I myself am returning to electronics as a hobby after a few (many) years away. Much to learn every day.

    But, I really don't understand the reference to "amplidynes old" ??

    The TIP29's are marginal in current handling for your project, but as you discoverd they are an improvement because they deliver enough current to make the motors run.

    Why don't you post the circuit as it is wired now. I suspect more suggestions might be helpful.

    Do you have some idea what the weight of your robot might be?
    How will you be controlling it? Microprocessor?, Radio Control?
     
  18. Adesoji Aderemi

    New Member

    Oct 27, 2012
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    Thanks. Its just that I don't know if amplidynes are still in use today for motor control. I can immagine there would be more effective electrical circuits now our days
     
  19. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Missed this on last post
    What happened to Peter Morley !?
     
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